Bromine biocide removal

ABSTRACT

A system and method are provided which removes bromine biocide used in effluent process streams without the use of chemicals or complex mechanical systems. In particular, the system and method of the present invention remove bromine biocide by photodissociating the bromine, thereby forming innocuous salts. Ultraviolet energy may be used to provide the energy for photodissociation, in particular ultraviolet energy supplied by medium pressure Hg vapor lamps.

FIELD OF THE INVENTION

[0001] This invention pertains to the treatment of industrial processwater, and more particularly, to the removal of bromine based biocidesin industrial process water streams. Specifically, the invention relatesto the removal of bromine based biocides from industrial process waterstreams using ultraviolet (UV) radiation.

BACKGROUND OF THE INVENTION

[0002] A wide variety of industries use water as the coolant for heattransfer processes. A significant quantity of cooling water is usedannually by both electrical power plants and manufacturing operations.Typical water cooled heat transfer processes include condensers andchillers. In general, the spent water is recycled to lessen both theeconomic and environmental impact of these heat transfer processes. Thisrecycling requires the spent water to be cooled back to ambienttemperature or slightly below, commonly by means such as a coolingtower. In general, cooling towers allow the spent water to release heatto the ambient air by cascading the spent water down an open air tower.

[0003] Cooling water, commonly supplied by a surface water source suchas a lake or stream, contains a wide range of contaminants. Thesecontaminants can lead to fouling and subsequent reduction of coolantflow and/or heat transfer efficiency. Minerals, such as calcium,magnesium and silica, can form scale deposits on the insides of pipesand tubes. Bacteria, carried into the cooling water system by wind ormake up water, can likewise foul process piping and heat exchangesurfaces by creating deposits, or, if left unchecked, a continuous layerof surface slime. Bacterial contamination is particularly troublesomebecause the heat transfer through heat exchanger surfaces can besignificantly reduced, corrosion can occur under the bacterial slimelayer, and the tacky nature of the slime layer provides a surface onwhich minerals can readily deposit.

[0004] To control the build up of these contaminants, industriestypically subject at least a portion of their process water stream,commonly referred to as a “blowdown stream, to a purification system. Ingeneral, process water purification systems are comprised of a series ofremoval processes, each designed to eliminate specific types ofcontaminants. For example, minerals may be removed from process waterstreams by methods such as deionization and reverse osmosis. Thepresence and growth of microorganisms may be controlled using methodssuch as oxidation, ultraviolet irradiation, and mechanical cleaning.Oxidation, the traditional method by which to eliminate microorganisms,remains popular today. Oxidizing biocides, such as chlorine and bromine,kill microbial growth by destroying important cellular components withinthe microorganisms.

[0005] Chlorine, first used to purify water in the 1800s, remains themost widely used oxidizing biocide. Chlorine provides its biocidalproperties by reacting with water to form hypochlorous acid, a verystrong oxidizing agent, in a pH sensitive reaction. However, althoughchlorine is an effective, easily applied biocide, it has severaldrawbacks. In 1982 the U.S. Environmental Protection Agency implementedregulations which effectively limited the use and the discharge ofchlorine. Further, high concentrations of halogenated biocides, such asthose required with chlorine biocides, are known to increase thecorrosion of carbon steel. Chlorine is particularly known to corrodecopper alloys, widely used in heat exchange equipment. Chlorine is alsoknown to degrade the wood fill used in cooling towers, leadingmanufacturers to use plastic fill. Further, chlorine is known to formundesirable reaction products with organics and ammonia compoundscommonly found in process water streams.

[0006] Bromine, introduced as a process water biocide in the 1940's, isan attractive alternative to chlorine. Bromine biocide may be applied toprocess water in several forms, including as a liquid solution of purebromine, as a bromine/chlorine mixture, or as a bromide-surfactantpackage. Recent product advances have resulted in biocide packagescontaining bromine granules with an equivalent molar ratio of chlorinefor quick activation and easy handling. The chlorine is included in thebiocide package to react with the bromine in water solution, thusproducing hypobromous acid and an innocuous chloride ion via thefollowing nonreversible reaction:

HOCl+Br⁻→HOBr+Cl⁻  (1)

[0007] The chloride ion subsequently bonds with available cations in thewater solution, such as sodium, to form salts, such as sodium chloride.The hypobromous acid which is formed is a strong oxidizing agent.Hypobromous acid is the active biocide formed by bromine, much ashypochlorous acid is responsible for chlorine's biocidal properties.However, in comparison, bromine dissociates into hypobromous acid muchmore effectively than chlorine dissociates into hypochlorous acid athigher pHs. In addition, the subsequent biocidal reaction of hypobromousacid is also highly effective at elevated pHs. The biocidal reaction ofhypobromous acid in water is:

HOBr+R₃CH→R₃COH+H⁺+Br⁻  (2)

[0008] In cooling towers in which the pH has been elevated with acaustic agent, such as NaOH, the hydronium ion (H⁺) and bromide ion(Br⁻) formed in the biocidal reaction (2) combine with the dissociatedions formed by the caustic agent, such as Na⁺ and OH⁻, to form a bromidesalt, such as Na⁺Br⁻, and water, H₂O. These reaction products, i.e.,bromide salt and water, are favored, thus driving the biocide reaction(2) to completion. The superior dissociative properties and biocidalactivity of bromine is particularly important in applications in whichcooling water is supplied by surface waters, which tend naturally tohave higher pHs and high nutrient levels for biogrowth.

[0009] In addition to greater oxidative efficacy at higher pHs, bromineoffers several other significant advantages over chlorine in processwater purification, including reduced corrosion to process piping andheat exchanger surfaces and lower vapor pressure. In particular, a lowervapor pressure biocide is advantageous because it reduces the rate ofloss of bromine to the atmosphere as the cooling water cascades downthrough the cooling tower packing.

[0010] Although the strong oxidizing power of bromine makes it a highlyeffective biocide, its oxidative properties have a detrimental impact onother water purification processes downstream. In particular, bromineattacks the deionization resins and reverse osmosis membranes typicallyused to remove inorganic and organic contaminants. Currently, bromine isremoved from process water streams by means of a reducing agent, such assodium bisulfite. Unfortunately, the use of chemical reagents to removebromine from a process stream is disadvantageous in that the mechanicalsystem, in particular the pump used to inject the reagent, canpotentially fail. Typical pump failures include a mechanical failure ofthe diaphragm of positive displacement pumps which are typically used, alack of proper priming on start up after chemical replenishment, poweroutages, or maintenance down time. Also, injection of a reducing agentis inefficient in purification systems because the compound produced bythe reaction of the reducing agent is typically removed downstream byother purification operations. For example, sulfite reaction products,such as those produced by the reaction of sodium bisulfite, must beremoved prior to discharging the process water stream back into naturalreceiving waters, such as a river, lake, or stream. Some aquaticorganisms, such as fish, are sensitive to sulfite reaction products,which can impair respiration (oxygen uptake) and liver function.Therefore, there remains a need in the art to convert the brominebiocide used in industrial process water streams into innocuouscompounds more reliably and efficiently.

SUMMARY OF THE INVENTION

[0011] The present invention provides a system and a method by whichbromine biocide may be removed from process water streams without theuse of either chemical reagents or complex mechanical systems, thusyielding a significantly more reliable, efficient means of removalwhich, in turn, provides greater protection for downstream purificationequipment. In particular, the present invention employs irradiation todecompose, or photodissociate, bromine biocide present in process watereffluent streams, thereby forming innocuous compounds. In a preferredembodiment, UV radiation is used to dissociate bromine biocide in aprocess water stream, thereby yielding innocuous bromide salts.

[0012] In one particularly advantageous embodiment, a water purificationsystem for process water containing bromine biocide is providedcomprised respectively of an irradiation chamber, a deionizer, and aseparation system for removing suspended solids. By irradiating theeffluent water stream, compounds less oxidative than the bromine biocideare formed, thereby protecting the downstream deionizer and separationsystem from degradation. In one aspect of this embodiment, theirradiation chamber is fitted with an UV light source, such as one ormore medium pressure mercury vapor lamps. The irradiation chamber mayfurther be comprised of a stainless steel cylindrical member, such as apipe, through which the effluent stream continuously flows. In itscylindrical embodiment, the working pressure in the irradiation chamberis up to 100 psi, typically 40 psi, the pressure loss is less than 2psi, and the mercury vapor lamp is rated generally from about 0.5 toabout 40 kW, and more advantageously from about 2 to about 5 kW. In oneadvantageous aspect of this embodiment, a dwell time of 10 to 20 secondsis provided in the cylindrical stainless steel irradiation chamber.

[0013] According to one aspect of the invention, the deionizer is a weakacid cationic exchange resin and the separation system is a reverseosmosis system. In a further embodiment, the water purification systemalso includes a secondary deionizer, such as a sodium zeolite ionexchange resin. In an alternate embodiment, a secondary system isprovided for bromine biocide removal, such as a pump for injecting areducing agent such as sodium bisulfite. Additionally, the waterpurification system can optionally include a dearator to remove gasestrapped in the deionized water. An alternative of this optionalembodiment utilizes an alkaline solution pump for elevating the pH ofsuch dearated waters, thereby ensuring that silica is kept insuspension. Typically, the effluent water stream is provided by acooling tower, and a pumping system is used to introduce the brominebiocide into the cooling tower feed stream. In one aspect thatembodiment, only a fraction of cooling tower water, i.e., a side stream,is treated in the water purification system.

[0014] Another aspect of the invention is to provide a method by whichto remove bromine biocide from a process water stream using irradiation,especially UV irradiation. In one particular embodiment, an effluentwater stream laden with bromine biocide is subjected to UV irradiationin an amount sufficient to form bromide anions. In one aspect of theinvention, the irradiation process consists of subjecting effluent watercontaining bromine biocide to UV energy in the range of about 200mWs/cm² to 3500 mWs/cm². The bromine biocide laden effluent watertypically contains from 0.1 mg/l to 3.0 mg/l bromine biocide. In oneexemplary embodiment, residual bromine biocide is not detectable in theeffluent water stream following the irradiation process of the presentinvention.

[0015] By photodissociating bromine biocide as an initial step in awater purification process, the system and method of the presentinvention protects downstream purification processes from oxidation morereliably and efficiently. Further, the system and method of the presentinvention are surprisingly robust, capable of treating water used innormal operations, which is generally quite dirty. Additionally, UVbased bromine biocide removal also makes the cooling tower blowdownstream more suitable for discharge to natural receiving waters such as ariver, lake or stream.

[0016] In a particularly advantageous embodiment, photodissociation isaccomplished by subjecting an effluent water stream to a sufficientquantity of UV radiation. The dissociated bromine biocide then formsless oxidative compounds, such as bromide salts, by combining withcations naturally occurring in the effluent water stream. These lessoxidative compounds do not attack ion exchange resins or reverse osmosismembranes, which may be present downstream. The present invention thusprovides a bromine biocide removal technique, which does not involvecomplex mechanical systems or the introduction of chemicals, which aresubsequently removed.

[0017] Further understanding of the processes and systems of theinvention will be understood with reference to the brief description ofthe drawings and detailed description which follows herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 represents a schematic drawing of a water purificationsystem in accordance with one embodiment of the present invention.

[0019]FIG. 2 is a cutaway plan view of an irradiation chamber providedin accordance with one embodiment of the present invention.

[0020]FIG. 3 is a plan view of an irradiation chamber provided inaccordance with one embodiment of the present invention.

[0021]FIG. 4 is a graphical representation of the data in Table 1.

[0022]FIG. 5 is a graphical representation of the data in Table 2.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention now will be described more fullyhereinafter with reference to the accompanying drawings, in whichpreferred embodiments of the invention are shown. This invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. Like numbers refer to like elements throughout.Further, the various control, monitoring, and electrical supply lineshave been omitted throughout to improve clarity of presentation andunderstanding.

[0024] Referring now to FIG. 1, a multi-component water purificationsystem is provided. The system receives an effluent water stream 10 thatcould be generated by any number of manufacturing processes, whichproduce water, which must be purified for reuse. In one advantageousembodiment of the present invention, the effluent water stream 10 isprovided as a blow down stream from a cooling tower.

[0025] Cooling towers are tall, open-air structures used to reduce thetemperature of large quantities of effluent water. Cooling towersfunction by cascading warm water (also referred to as spent water) overa tower containing slat gratings or chevron packing. The slat gratings,or chevron packing, provide surface area for liquid/air contact, and aregenerally constructed of wood or plastic. The cooled water is collectedin a concrete or metal basin at the foot of the tower, and may then bepumped back into the heat transfer process for reuse.

[0026] The purpose of cooling towers is to provide a “loop system,” inwhich process water can be recycled indefinitely. However, water lossesdo occur, in great part due to evaporation. In addition, contaminantswhich build up in the system over time are typically removed via a“blowdown stream.” The blowdown stream diverts a fraction of the coolingwater, generally 5 to 20%, from the bottom of the basin. The watercomprising the blow down stream may be conserved by subjecting the blowdown stream to purification processes and returning the purified waterto the cooling tower. Therefore, in a preferred aspect of the invention,a cooling tower blow down stream supplies the effluent water stream tobe purified.

[0027] As noted earlier, industrial process water in general, andcooling tower water in particular, contains a wide variety ofcontaminants. These impurities include algae, bacteria, and organicswhich enter the system from the atmosphere during the cooling process;calcium, magnesium, and silica present in the incoming water source; andphosphonates which are added to keep silica in suspension. Biocide istypically injected into the cooling tower water to kill the algae andbacteria. In the present invention, a bromine biocide is used.Deionization processes 14, 28, particularly those comprising ionexchange resins, are then used to remove calcium and magnesium cationsfrom the process water. Techniques such as reverse osmosis 16 are usedto separate out solids such as silica, organics, and phosphonate fromdeionized water. Due to its high oxidation potential, the brominebiocide used to eliminate bacteria is harmful to these downstreampurification processes, in particular the ion exchange resin and reverseosmosis membranes, and must be substantially removed.

[0028] Bromine, biocides are available in several forms, including purebromine, bromine/chlorine mixtures, or as a bromine-surfactant package.The bromine biocide preferably employed in the present invention is purebromine in its diatomic form or bromine compounded with not more than anequivalent molar ratio of chlorine. Pure bromine may be added by anymeans known in the art. Further, the bromine biocide may be added atseveral points within the cooling tower. For example, bromine may beinjected as a side stream to the inlet water stream feeding the coolingtower. Conversely, bromine may be supplied to the system by placingpellets in the cooling tower basin or as granules in a side stream flowthrough cannister.

[0029] Typical concentrations of bromine biocide employed in theeffluent stream range from about 0.2 to up to 3 mg/L, more typicallyranging from 0.2 to 0.5 mg/L. As is known in that art, shock treatmentsof biocide are occasionally required, such as on a semi-annual basis.Typical concentrations of bromine biocide present during shock treatmentrange from 3.0 to 5.0 ppm. Although referred to as “pure” bromine, aminor amount of chlorine, such as 0.01 to 0.1 ppm, must be added to thecooling tower water to activate the bromine. The chlorine may be addedas a compound with the bromine or separately as either gaseous chlorineor sodium hypochlorite, as is known in the art.

[0030] The present invention removes bromine biocide by irradiating theeffluent water stream 10 with electromagnetic radiation. Such requisiteelectromagnetic radiation is supplied to the effluent water stream 10 asit passes through a radiation chamber 12. In a particularly advantageousembodiment, the present invention bombards the bromine biocide withultraviolet radiation, generally defined as electromagnetic radiationhaving wavelengths ranging from approximately 100 to 3900 angstroms (Å),or 10 to 390 nm. Broadband UV radiation, in the wavelength range of 240to 300 nm may be used. Potentially, narrower band sources providingsubstantial output in the 185.0 or 253.7 nm range may have someapplicability.

[0031] Referring now to FIGS. 2 and 3, an irradiation chamber 12 inaccordance with a particularly advantageous embodiment of the inventionis provided. FIG. 2 provides a cut away view of an irradiation chamber12 in accordance with one embodiment of the present invention. FIG. 3schematically illustrates further aspects of an irradiation chamber 12of the present invention, comparable to the irradiation chamber providedin FIG. 2.

[0032] Turning first to FIG. 2, the irradiation chamber 12 generallycomprises a cylindrical member 42, having opposing ends 60, 62. Ingeneral, the purpose of the irradiation chamber 12 is to illuminate theincoming effluent water stream 10 with a UV light source 40, which ispreferably protected from the effluent stream by a UV transparent“window.” The transparent window provided in the embodiment of FIG. 2comprises a high purity cylindrical quartz sleeve 54 running the entirecenter length of the chamber. The UV light source 40 and the quartzsleeve 54, define the longitudinal axis of the cylindrical member 42 andare mounted in concentric, parallel relation to the cylindrical member42, such that the UV light source 40 most effectively illuminates theeffluent water stream 10 as it travels through the irradiation chamber12. In the embodiment provided in FIG. 2, the effluent water stream 10immerses the quartz sleeve 54 as the stream flows through theirradiation chamber 12. The quartz sleeve 54 is supported at opposingends of the irradiation chamber 12 by support members 58. The UV lightsource 40 is preferably secured within the UV transparent window 54 by aprotective collar 56.

[0033] Suitable sources of UV radiation include Hg (mercury) vapordischarge lamps and xenon flash lamps. In one particularly advantageousembodiment, medium pressure Hg vapor lamps, producing a preponderance ofradiation of wavelengths in the 3000-6000 Å range, are employed. Lowpressure Hg vapor lamps, producing primarily 2537 Å radiation, may alsobe of benefit. In general, Hg vapor lamps rated from about 0.5 to about40 kW may advantageously be employed in the present invention, andpreferably from about 2 to about 5 kW. In one particularly advantageousembodiment, a single medium pressure Hg vapor discharge lamp, rated at 2kW, is used to treat a 6 to 8 gpm effluent stream. In alternativeembodiments, effluent streams of up to 100 gpm or more may be treatedusing lamps rated up to 40 kW. In further alternative aspects of thepresent invention, a multiplicity of UV lamps are utilized.

[0034] Turning to the embodiment shown in FIG. 3, the irradiationchamber 12 is formed from a cylindrical member 42, such as stainlesssteel tubing. In particular, the cylindrical member 42 may be formedfrom 316 L stainless steel tubing. The cylindrical member 42 is fittedwith inlet and outlet flanges at its opposing ends, shown respectivelyas 44 and 46, Such flanges may be of any suitable dimension inporportion to the cylindrical member 42. In one preferred embodiment,such flanges are 3 inches in diameter. In general, the maximum workingpressure in the irradiation chamber 12 is 100 psi. In one particularembodiment, the working pressure within the irradiation chamber 12 is 38psi (206 Bar), and the pressure loss at maximum flow is less than 2 psi(138 mBar). Optionally, the irradiation chamber 12 may be fitted with aUV monitor 50 and a temperature sensor 52. UV irradiation chambers asdescribed above are available commercially from Aquionics as Model No.UV500XP, which provides a 150 mm diameter cylindrical irradiationchamber having a 435 mm flow path.

[0035] Although a particular configuration has thus far been described,irradiation chambers having a multitude of configurations are possible,so long as sufficient energy is supplied to the effluent water flowingthrough the chamber. The amount of energy required may be determined fora given effluent stream empirically by testing the irradiated stream forresidual bromine by methods known in the art, such as titration, andadjusting effluent flow rates and energy input accordingly. Brominebiocide is considered to be effectively removed when the residual levelof bromine present in the irradiated water stream is less than or equalto 0.1 mg/l. As a general rule, it would be expected that sufficientenergy would be imparted to an effluent water stream containing from 0.1to 0.3 ppm bromine biocide by exposure intensities ranging from 200 to350 mWs/cm², preferably 300 mWs/cm² (referenced to distilled water) fora dwell time of 10 to 20 seconds, preferably 15 seconds. However, as aadditional note, the UV light source is preferably sized so as to havethe ability to effectively treat bromine spikes of 3 mg/L commonly usedfor semiannual shock treatments. To effectively treat effluent waterstreams containing a bromine spike of up to 3 mg/L, UV exposureintensities from 2000 to 3500 mWs/cm², preferably 3000 mWs/cm²(referenced to distilled water) are recommended for a dwell time of 10to 20 seconds, preferably 15 seconds. These dosages are anticipated fora typical cooling tower blowdown stream in an otherwise maintainedsystem whose water stream ranges in color from green to greyish andwhich has a visibility to several feet in depth. The actual UV dosageapplied to bromine biocide present in the turbid water normallyencountered in commercial process operations is projected to be ½ to ¼the applied intensities given above for distilled water. The UV dosagerate may be varied by adjusting the flow rate through the UV unit. TheUV dosage rate may be determined using the following formula:

D=I×t _(d)  (3)

[0036] Where

[0037] D is dosage rate in mWs/cm²

[0038] I is exposure intensity in mW/cm²

[0039] t_(d) is dwell time in seconds.

[0040] The dwell time may further be calculated by dividing the exposedliquid volume by the flow rate.

[0041] In general, UV energy is known in the art for use in waterpurification. As an example, UV has been used for years in the treatmentof drinking water to remove bacteria, as taught in U.S. Pat. No.5,679,257. UV is also known to have a beneficial impact on macrofouling,in particular in controlling zebra mussels and other bivalves, asdiscussed in U.S. Pat. No. 5,655,483. Chlorine is known to be activatedby radiation in the range of 3000 to 6000 Å, as described in U.S. Pat.No. 4,402,836, directed to ultraviolet induced chlorination ofhydrazine-fuel contaminated wastewater. Further, UV has been used tophotodissociate halogen containing gaseous molecules. Such a use ofphotodissociation is described in U.S. Pat. No. 5,534,107, in whichgases containing chlorine or bromine moities are dissociated using UV.

[0042] UV has also been touted as an effective method by which chlorinemay be removed from effluent water streams and testing of suchapplications is ongoing. However, although UV can arguably removechlorine from otherwise “clean” effluent water streams, it has not beenproven effective when used as a primary means of chlorine removal forthe “dirty” water encountered in many commercial processes, such as acooling tower blowdown stream. For further discussion see Richard Combsand Norman Ammerer, Chlorine Removal-Ultraviolet Light Oxidation OfChlorine In Water, 15 ULTRAPURE WATER®, The Definitive Journal ForIndustrial Water Users, 21 (No. 4, April 1998), hereby incorporated byreference in its entirety. This result may not be altogether surprising,in light of the fact that UV treatment is not recommended for thetreatment of turbid water in other applications. In that regard, seeU.S. Pat. No. 5,679,257. As noted previously, process water,particularly process water recirculated through a cooling tower, can bequite dirty, containing a range of contaminants, such as high loadingsof organics, mineral cations, silica and phosphonates. It is known thatcooling tower blowdown streams may contain visible green biogrowth andgreyish suspended matter, suspended solids, dissolved solids, and veryhigh levels of bacteria. Suspended particles may be present in amountsof 150,000 particles/liter or more. These particles may range in sizefrom 5 to 500 μm, with the majority of particles less than 25 μm. Table1, shown below, provides a typical suspended solids analysis for acommercial cooling tower stream. In addition, dissolved solids inamounts exceeding 700 mg/l are known. Bacteria may be present in theblowdown stream at 10³ counts per milliliter when effective amounts ofbiocides are used and at greater than 10⁷ counts per milliliter whenbiocides are ineffective. The presence of these contaminants give riseto a turbid appearance in the effluent water stream. TABLE 1 COOLINGTOWER SUSPENDED PARTICLE ANALYSIS Particle Size No. of ParticlesParticle Size Range (μm) (count/100 ml) Distribution (%)  5 μm to 158623 57.0 15 μm to 25 4361 28.8 25 μm to 50 1798 11.9  50 μm to 100 2962.0 100 μm to 200 12 <0.5 200 μm to 300 8 <0.5 300 μm to 400 6 <0.5 400μm to 500 2 <0.5 >500− 20 <0.5

[0043] It is believed that the irradiation process removes brominebiocide from the effluent water stream by photodissociation, i.e., theUV energy causes the bromine present in the hypobromous acid and/orbromine to dissociate into bromide ions. These bromide ions subsequentlycombine with cations in the effluent water stream, thereby forminginnocuous bromide salts. Although not wishing to be bound by theory, itis hypothesized that the superior results achieved with bromine biocideis due to the fact that hypobromous acid in solution has a loweractivation energy than hypochlorous acid in alkaline water, i.e. waterhaving a pH>8. This lower activation energy at elevated pHs is believedto be due to the favorable ultimate reaction products, namely bromidesalt and water, formed by the hydronium ion and bromide ions producedduring the biocidal reaction (2). In the alternative, it is theorizedthat bromine and/or hypobromous acid may have a lower bond strength thaneither hypochlorous acid and/or chlorine. It is further hypothesizedthat bromine and/or hypobromous acid may absorb UV energy moreefficiently than hypochlorous acid and/or chlorine. It is believed thatany of the inherent molecular differences such as those described abovemay account for the difference in results noted for the UV irradiationof bromine biocide in comparison to its effectiveness with chlorinebiocides.

[0044] As noted earlier, the removal of hypobromous acid, a strongoxidizing agent, is highly beneficial for downstream processingequipment. Returning now to FIG. 1, a typical water purification system,comprised of a multi-step process, is shown in which bromine biocide maybe effectively removed before it encounters subsequent sensitivepurification processes.

[0045] One such sensitive process is a deionizer 14, illustrated astreating water exiting the irradiation chamber 12. As is known in theart, deionizers generally remove charged particles from a solution bycontacting the solution with ion exchange resins. In the presentinvention, cations, such as the Ca⁺ and Mg⁺ ions naturally present inhard water, are removed from the effluent water stream. The amount ofCa⁺ and Mg⁺ ions present in water is commonly referred to as itshardness. In one aspect of this embodiment, the hardness removal isaccomplished with a weak acid cationic resin.

[0046] To provide additional purification, a secondary deionizer 28 mayoptionally be employed. Typical secondary deionizers may be comprised ofresins such as sodium zeolite. The purpose of the secondary deionizer isto remove any stray Ca⁺ and/or Mg⁺ cations which were not removed by theprimary deionizer 14. In one embodiment, the secondary deionizer is abed of sodium zeolite ion exchange resin.

[0047] The primary ion exchange process drops the pH of the deionizedwater stream. This decrease in pH leads to the formation of CO₂. Adearator 24 is provided to remove alkalinity from the deionized waterstream. As is known in that art, a dearator is generally comprised of anenclosed, elongated vessel in which liquid containing entrained gases isintroduced into the top section of the vessel via a series of spraynozzles. The liquid is allowed to free fall into the bottom section ofthe vessel, where it is subsequently pumped out. During free fall, gasestrapped within the liquid are released and discharge vertically into thehead space above the dearated liquid. As known in the art, the liquidmay be distributed with gravity trays rather than nozzles, and may fallover plastic packing balls of various sizes and shapes, rather than freefall. Other advances in dearation, such as enclosing the vessel andpulling a vacuum on the gas, and/or applying a membrane between theliquid and the vacuum may be employed. Further, the use of sparging orsweep gas is known.

[0048] In the embodiment illustrated in FIG. 1, the dearated water issubjected to a reverse osmosis process 16 to separate out dissolvedsolids in the effluent water stream. Reverse osmosis processes are wellknown in the art. Reverse osmosis generally involves the use of highpressure to force a solution through a semipermeable membrane which hasbeen designed to allow only the solvent to pass through. In the presentinvention, effluent water containing dissolved solids such as silica andbromide salts, is forced through thin film semipermeable membranes. Suchthin film semipermeable membranes are described in U.S. Pat. No.5,925,255 to Mukhopadhyay, hereby incorporated by reference. In theembodiment in FIG. 1, a series of semipermeable membranes forms thereverse osmosis system. In particular, a single section 4 inch diametersemipermeable membrane 18 is followed by two sections of 2½ inchdiameter semipermeable membrane 20. Each section is 120 inches long,comprised of three 40 inch membranes of a given diameter, joined end toend. These membranes are designed to filter out 100% of suspendedparticles and the majority of dissolved solids. In a preferredembodiment of that aspect, a pump 32 is used to apply about 500 psigpressure to the effluent water stream entering the reverse osmosisprocess. To conserve the effluent water stream, concentrated solutionmay optionally be recirculated continuously within the reverse osmosisprocess 16, thereby ensuring optimal removal of process water from thesludge stream.

[0049] To avoid premature fouling of the semipermeable membranes, it isconsidered beneficial to maintain silica in suspension during thereverse osmosis process. It is widely known in the art that silicasolubility is directly proportional to the pH and that silica may bemaintained in aqueous solutions by raising the pH. Therefore, in oneembodiment of the present invention, an alkaline solution system 26 isprovided to maintain and inject alkaline solution into the effluentstream as appropriate. In one aspect of this embodiment, a sufficientamount of a 25% by weight sodium hydroxide solution is added to theeffluent water stream to maintain a constant pH of approximately 11.

[0050] In a further embodiment of the present invention, a system forinjecting a reducing agent into the effluent stream 22 is provided. Sucha reducing agent system may be useful to provide additional brominebiocide removal during shock treatments, as well as for use in periodsin which the irradiation chamber 12 has been taken out of service. Inone aspect of this embodiment, a 2.4% by weight sodium bisulfitesolution is used as the reducing agent. A quantity of sodium bisulfitesolution sufficient to eliminate any residual bromine biocide is addedby means such as a pump, as is known in that art.

[0051] Polishing filters 30 may also be utilized to remove fineparticles suspended in the effluent water stream. In particular, one ormore 1 micron filters formed from poly(ethylene) terephthalate fiber maybe of benefit for use in the present invention.

[0052] Following the reverse osmosis process 16, the purified water isreturned to the process water stream 34 for reuse. Only highlyconcentrated sludge is sent to the sewage system 36. Under typicaloperating conditions, about 90% of the water supplied by the effluentstream 10 is returned to the system as purified water 34 and about 10%is disposed of in the sewer 36. Therefore, under typical operatingconditions, for a cooling tower supplying a 10 to 40 gpm blow downstream to the water purification process, approximately 9 to 36 gpmwater would be recycled, while only 1 to 4 gpm water would requireneutralization and discharge to the sewer or an evaporation pond.

[0053] By providing a more reliable method of bromine biocide removal,the useful life of both the deionization and reverse osmosis processescommonly utilized in water purification systems is extended. Thesepurification processes, along with the bromine biocide which mustultimately be removed from the system, allow enormous quantities ofprocess water to be recycled indefinitely. Therefore, the moreefficient, reliable method of bromine biocide removal of the presentinvention provides significant economic and environmental benefits.

[0054] The present invention will be further illustrated by thefollowing non-limiting examples.

EXAMPLE 1

[0055] Samples 1 through 11 were purified using a pilot plant waterpurification system having a configuration comparable to that providedin FIG. 1. Namely, the pilot plant purification system consisted of anirradiation chamber, a primary ion exchange resin system, a dearator, asecondary ion exchange system, a sodium hydroxide pump, and a reverseosmosis process. Samples were irradiated in a model UV500XP irradiationchamber from Aquionics Company, using a 2 kW medium pressure Hg vaporlamp, applying a dosage level of from about 2000 to about 3000 mWs/cm²referenced to distilled water. As noted previously, the effective dosageimparted to the bromine biocide present in commercial process waterstreams is expected to be one half to one fourth the applied dosagereferenced to distilled water, due to the turbidity encountered in suchprocess water streams. This turbidity is primarily due to suspendedsolids and other contaminants. The primary ion exchange system contained20 ft³ of a weak acid cationic resin. The dearator was rated at 500gallon capacity. The secondary ion exchange system contained 3.5 ft³ ofsodium zeolite resin. The pH entering the reverse osmosis process wasmaintained at about 11 using a NaOH solution. The reverse osmosisprocess consisted of 120 inches of 4″ diameter thin film membranefollowed by two sections of 120 inches of 2.5″ diameter thin filmmembrane. These thin film membranes were generally comprised ofpolyamide material, and were pervious to water, but impervious tosuspended solids and the majority of dissolved solids. The typicalrejection for dissolved solids is 90%.

[0056] The bromine biocide added to the system was a chlorine/brominemixture from Great Lakes Water Treatment, tradename Bromicide™, added asa 0.2% by weight solution using granules placed in a 1 cu. ft.sidestream bottle. The bromine was activated with an equivalent molarratio of chlorine, included in the granules. The bromine biocide wasadded to water typical of that found in cooling tower operation andcommonly considered “dirty” water. Namely, the water used in the sampleshad a pH of 8.3 and above; Total Dissolved Solids (TDS) values from 503to 716 mg/l; total hardness from 250 to 385 mg/l; total organic carbon(TOC) from 9.14 to 15.7 mg/l; and silica levels from 130 to 152 mg/l.The residual bromine remaining in the sample following irradiation wasdetermined by a LAMOTT's bromine color comparator (DPD method) with arange of 0.1 to 3.0 mg/L from samples taken immediately following theirradiation process. The results are provided in Table 2 and graphicallyillustrated in FIG. 4. As shown by the results in Table 2 and FIG. 4,bromine biocide can be effectively removed at effluent flow rates of upto 8.4 gpm using a single 2 kw mercury vapor discharge lamp.

COMPARATIVE EXAMPLES

[0057] Comparative Examples, samples 12, 13, 14 and 15 are provided inTable 3, indicating the effectiveness of UV irradiation as adechlorination technique for an IPA scrubber tower effluent stream. TheComparative Examples were prepared using full scale equipment similar tothe pilot plant UV unit described in Example 1. The UV irradiationchamber was a UV850XXP by Aquionics with a medium pressure UV lamp ratedat 5.0 KW, which applied a UV dosage of about 1575 mWs/cm² to waterflowing through UV unit at a rate of about 81 gpm. Chlorine biocide inthe form of sodium hypochlorite was added to the inlet water stream. InSamples 12, 13, 14 and 15 “clean” city water was used as makeup to anIPA scrubber during testing. The water irradiated in Samples 12-15 had aTDS of 68 mg/l and IPA at 1500 mg/l. Chlorine concentration wasdetermined using a HACH DR100 calorimeter test kit, DPD method.

[0058] The results are provided in Table 3 and graphically illustratedin FIG. 5. A comparison of Samples 12, 13, 14 and 15 indicates that UVradiation is not effective at removing the chlorine biocide contained ineven this relatively clean water. In particular, as shown in Sample 14,the minimum residual chlorine level achieved for the ComparativeExamples was an unacceptable 0.10 mg/l. Further, these ComparativeExamples represent a “best case” scenario because the water used in theComparative Examples was relatively “clean,” having a TDS of only 68mg/l. As noted earlier, the presence of dirty or turbid water, having bydefinition high solids levels, is known to have a detrimental impact onthe effectiveness of UV treatment. Therefore, these results indicatethat UV irradiation would not be an effective means by which to removechlorine from normal process water streams, which typically contain muchhigher solids levels than the Comparative Examples.

[0059] In contrast, the results provided in Table 2 and FIG. 4 indicatethat UV irradiation is effective for removing bromine biocide fromprocess water streams comparable to normal operating conditions. Infact, bromine biocide at concentrations of up to 2.2 mg/l have beeneffectively removed from water having a minimum TDS of 503 mg/l, asolids level comparable to the turbidity of commercial process water.Detectable residual bromine biocide remains only in Samples having inletbromine biocide concentrations comparable to “spike” conditions, i.e.,those samples having an inlet concentration of 2.8 mg/l or more. It isbelieved that higher UV dosage levels would eliminate residual brominebiocide even during these “spike” conditions. Therefore, UV irradiationhas been found to be an effective means by which to remove brominebiocide from process water streams used in normal facilities operations,such as scrubbers and cooling towers.

[0060] Many modifications and other embodiments of the invention willcome to mind to one skilled in the art to which this invention pertainshaving the benefit of the teachings presented in the foregoingdescriptions and the associated drawings. Therefore, it is to beunderstood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation. TABLE 2BROMINE BIOCIDE REMOVAL BY UV IRRADIATION FOR COOLING TOWER BLOWDOWNBromine Biocide Bromine Biocide In Effluent Feed After Effluent FlowrateSample Stream (mg/l) Irradiation (mg/l) (gpm) 1 1.5 0 6.05 2 1.7 0 6.703 1.8 0 6.60 4 2.2 0 6.60 5 2.8 0.1 6.70 6 3.0 0 7.70 7 1.7 0 6.55 8 3.00.1 8.4 9 3.0 0.1 8.4 10  3.0 0.2 7.3 11   1.25 0 6.55

[0061] TABLE 3 CHLORINE BIOCIDE REMOVAL BY UV IRRADIATION AT IPASCRUBBER TOWER Chlorine Biocide Chlorine Biocide In Effluent Feed AfterSample Stream (mg/l) Irradiation (mg/l) 12 1.5 0.4  13 1.5 0.15 14 1.00.10 15 0.6 0.15

That which is claimed:
 1. A water purification system comprising: anirradiation chamber for treating bromine biocide present in an effluentwater stream, thereby forming compounds which are less oxidative thansaid bromine biocide; a deionizer for removing hardness in the form ofcations present in said irradiated water; and a separation system forremoving suspended and dissolved solids from said deionized water;whereby said formation of compounds which are less oxidative than saidbromine biocide protects said deionizer and said filtration system fromdegradation by said bromine biocide.
 2. The water purification system ofclaim 1, wherein said bromine biocide is initially present in an amountranging from about 0.1 mg/l to 3.0 mg/l and said irradiation chambertreats said bromine biocide with an intensity of ultraviolet radiationin the range of about 200 mWs/cm² to 3,500 mWs/cm².
 3. The waterpurification system of claim 2, wherein less than 0.1 mg/l residualbromine biocide is present following said treatment in said irradiationchamber.
 4. The water purification system of claim 1, wherein saidirradiation chamber further comprises an ultraviolet light source. 5.The water purification system of claim 4, wherein said ultraviolet lightsource is a single medium pressure mercury vapor lamp.
 6. The waterpurification system of claim 4, wherein said ultraviolet light sourcecomprises a multiplicity of medium pressure mercury vapor lamps.
 7. Thewater purification system of claim 5, wherein said irradiation chamberfurther comprises a stainless steel cylindrical member having an inletfor allowing untreated water to enter chamber and an outlet for allowingirradiated water to exit said chamber.
 8. The water purification systemof claim 7, wherein said stainless steel cylindrical member provides adwell time of 10 to 20 seconds.
 9. The water purification system ofclaim 7, wherein the working pressure in said irradiation chamber is nogreater than 100 psi, the pressure loss at maximum flow is less than 2psi, and said mercury vapor lamp is rated from about 0.5 to about 40 kW.10. The water purification system of claim 9, wherein said mercury vaporlamp is rated from about 2 to about 5 kW.
 11. The water purificationsystem of claim 1, wherein said deionizer comprises a weak acid cationicion exchange resin for removing hardness.
 12. The water purificationsystem of claim 1, wherein said separation system comprises a reverseosmosis system.
 13. The water purification system of claim 1, furthercomprising a pump for injecting a reducing agent into said irradiatedwater.
 14. The water purification system of claim 13, wherein said pumpinjects sodium bisulfite.
 15. The water purification system of claim 1,further comprising a dearator to remove gases trapped in said deionizedwater.
 16. The water purification system of claim 15, further comprisingan alkaline solution pump for elevating the pH of said dearated water.17. The water purification system of claim 16, further comprising: acooling tower providing said effluent water stream; and a brominebiocide pump to introduce bromine biocide into a cooling tower feedstream.
 18. The water purification system of claim 17, wherein saidcooling tower diverts 5 to 20% of its total volume of water, therebyproviding said effluent water stream.
 19. The water purification systemof claim 17, further comprising a secondary deionizer for hardnessremoval.
 20. The water purification system of claim 19, wherein saidsecondary deionizer for hardness removal comprises a sodium zeolite ionexchange resin.
 21. A water purification method comprising: irradiatingbromine biocide contained in an effluent water stream; allowing saidirradiated bromine to react with cations contained in said effluentstream, thereby forming compounds which are less oxidative than saidbromine biocide; removing ions from said effluent water stream followingformation of said less oxidative compounds to merely soften the water;and separating suspended and dissolved solids from said softened water,whereby said formation of compounds which are less oxidative than saidbromine biocide protects the apparatus used in said ion removal andseparating steps from degradation by said bromine biocide.
 22. Themethod of claim 21, wherein said irradiating step comprises exposingsaid bromine biocide contained in said effluent water stream toultraviolet light.
 23. The method of claim 22, wherein said irradiatingstep comprises exposing said bromine biocide to ultraviolet radiation inan amount sufficient to form bromide anions.
 24. The method of claim 23,wherein said irradiating step further comprises exposing said brominebiocide to an intensity of ultraviolet radiation in the range of about200 mWs/cm² to 3,500 mWs/cm².
 25. The method of claim 21, wherein saidstep of removing ions comprises subjecting said effluent water to an ionexchange process.
 26. The method of claim 25, wherein subjecting saideffluent water to said ion exchange process comprises contacting saideffluent water with a weak acid cationic ion exchange resin.
 27. Themethod of claim 21, wherein said step of separating comprises subjectingsaid deionized water to a reverse osmosis process.
 28. The waterpurification method of claim 27, further comprising raising the pH ofsaid effluent water stream prior to entering said reverse osmosisprocess.
 29. The water purification method of claim 21, furthercomprising dearating said deionized water.
 30. The water purificationmethod of claim 21, further comprising: subjecting said deionized waterto a secondary deionization process; and reacting residual brominebiocide present in the irradiated effluent water stream with a reducingagent.
 31. The water purification method of claim 21, further comprisingproviding a bromine biocide laden effluent water prior to saidirradiating step, whereby said bromine biocide is supplied in an amountranging from about 0.1 mg/l to 3.0 mg/l.
 32. The water purificationmethod of claim 31, wherein said allowing step permits said irradiatedbromine biocide to react to an extent such that less than 0.1 mg/lresidual bromine biocide is present in said separated water.